Learn DTOs and Domain Models | Clean Architecture for API Apps

When building Flutter apps that interact with REST APIs, you often receive data in the form of raw JSON. However, letting your UI code depend directly on this JSON, or even on data transfer objects (DTOs) that mirror the API, is risky and limiting. Instead, you should introduce two layers: DTOs and domain models.

What is a DTO?

A DTO (Data Transfer Object) is a simple class that represents the data structure exactly as it comes from the API. It is designed for serialization and deserialization, making it easy to convert between Dart objects and JSON. DTOs are often tightly coupled to the API contract and may include nullable fields, naming conventions, or types that match the backend.

What is a Domain Model?

A domain model represents the core concepts and business logic of your app, independent of how the data is fetched or stored. Domain models are what your UI and business logic should use. They are shaped according to your app’s needs, not the backend’s structure. By mapping DTOs to domain models, you shield your app from backend changes and keep your UI logic clean and focused.

main.dart


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// DTO representing the API response for a user
class UserDto {
  final int id;
  final String? first_name;
  final String? last_name;
  final String? email;

  UserDto({
    required this.id,
    this.first_name,
    this.last_name,
    this.email,
  });

  factory UserDto.fromJson(Map<String, dynamic> json) {
    return UserDto(
      id: json['id'] as int,
      first_name: json['first_name'] as String?,
      last_name: json['last_name'] as String?,
      email: json['email'] as String?,
    );
  }
}

// Domain model representing a user in the app
class User {
  final int id;
  final String fullName;
  final String email;

  User({
    required this.id,
    required this.fullName,
    required this.email,
  });
}

// Mapper function to convert a UserDto to a User domain model
User userFromDto(UserDto dto) {
  final firstName = dto.first_name ?? '';
  final lastName = dto.last_name ?? '';
  return User(
    id: dto.id,
    fullName: (firstName + ' ' + lastName).trim(),
    email: dto.email ?? '',
  );
}

// Example usage
void main() {
  // Simulated API response
  final json = {
    'id': 42,
    'first_name': 'Ada',
    'last_name': 'Lovelace',
    'email': 'ada@example.com',
  };

  // Parse DTO from JSON
  final userDto = UserDto.fromJson(json);

  // Map DTO to domain model
  final user = userFromDto(userDto);

  print('User domain model:');
  print('ID: [1m${user.id}[0m');
  print('Full Name: [1m${user.fullName}[0m');
  print('Email: [1m${user.email}[0m');
}

By separating DTOs from domain models, you gain important flexibility. Your UI and business logic can work with clean, well-defined models that fit your app's needs, rather than being forced to match the backend's data structure. If the API changes, you only need to update the DTO and mapping layer, not your entire app.

Note

This also lets you add computed fields, validation, or transformations in the mapping step.

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Section 3. Chapter 2

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